Small mobile devices, such as digital cameras, mobile phones and portable game devices, that are equipped with sensors, such as angular velocity sensors and acceleration sensors, have become widespread recently. With this trend, there is a strong demand for small-sized sensors having smaller footprint, lower profile and higher accuracy. Patent literature 1 discloses a sensor including a sensor chip and a circuit chip stacked and accommodated in a ceramic package to reduce a size of the sensor.
FIG. 21 is a schematic sectional view of conventional sensor 50, an angular velocity sensor described in Patent Literature 1. Sensor chip 55 is stacked on circuit chip 53 via film adhesive 54. Circuit chip 53 is fixed to an upper surface of ceramic package 51 with adhesive 52. Sensor chip 55 and circuit chip 53 are electrically connected with bonding wire 56. Circuit chip 53 and ceramic package 51 are connected electrically with bonding wire 57. This stacked structure in which sensor chip 55 and circuit chip 53 are stacked provides sensor 50 with a small size. Sensor 50 detects an angular velocity as an electrical signal converted from a Coriolis force generated when a moving object rotates, due to the angular velocity exerted upon it. An analog detection signal of the Coriolis force detected by sensor chip 55 undergoes a predetermined digital-signal processing in circuit chip 53, and it is output as an angular velocity detection signal. A mobile device equipped with the angular velocity sensor executes various processes by using this angular velocity detection signal.
In conventional sensor 50, electrodes of sensor chip 55 and circuit chip 53 are connected via bonding wire 56 since they cannot be connected directly due to a difference in areas of their surfaces. In general, the analog detection signal output from sensor chip 55 to circuit chip 53 is a very small signal. The analog detection signal input from sensor chip 55 is converted into a digital signal, and a necessary digital signal processing is carried out in circuit chip 53 to produce an angular velocity detection signal. The angular velocity detection signal transmits as a digital signal from circuit chip 53 to ceramic package 51 through bonding wire 57. For this reason, a high-frequency digital noise is radiated from bonding wire 57, which causes an adverse effect on the analog detection signal flowing through bonding wire 56. This effect of the digital noise becomes more remarkable as increase in lengths of bonding wires 56 and 57. The digital signal in circuit chip 53 may intrude directly into sensor chip 55 as a noise, and adversely affects the analog detection signal in sensor chip 55. Furthermore, a noise generated outside of sensor 50 may cause an adverse influence directly upon the analog detection signal. These effects may decrease detecting accuracy of the angular velocity sensor.
FIG. 22A is an exploded perspective view of another conventional sensor 501 disclosed in Patent Literature 2. Sensor 501 is known as an IC-integrated acceleration sensor. FIG. 22B is a cross-sectional view of sensor 501 shown in FIG. 22A. Sensor 501 includes sensor chip 502, IC chip 503 for electrically processing a detection signal from sensor chip 502, protective case 504 accommodating sensor chip 502 and IC chip 503, and cover 504a that hermetically seals an inner space of protective case 504.
Sensor chip 502 includes flexible portion 502a, weight 502b, and support body 502c. A piezo-resistance element is disposed onto flexible portion 502a for converting acceleration exerted on sensor chip 502 into an electric detection signal. A row of sensor chip terminals 505 is formed on an upper surface along one side edge of support body 502c. 
IC chip 503 includes a processing circuit and chip terminals 506 formed on its main surface. When the main surface is oriented upward, IC chip 503 is bonded to the upper surface of support body 502c of sensor chip 502 with a predetermined space to the upper surface of support body 502c with adhesive 507a containing fine rigid plastic balls, thereby restraining a movable range of flexible portion 502a and weight 502b of sensor chip 502.
The lower part of support body 502c is also bonded to an upper surface of protective case 504 with a predetermined space with adhesive 507b containing fine rigid plastic balls, so as to restrain a vibration range of weight 502b of sensor chip 502.
Some of chip terminals 506 are electrically connected with sensor chip terminals 505 through wires 508a, and the other of chip terminals 506 are electrically connected with protective case terminals 509 through wires 508b. 
In conventional sensor 501, wires 508a connecting chip terminals 506 with sensor chip terminals 505, and wires 508b connecting chip terminals 506 with protective case terminals 509 protrude above the main surface of IC chip 503 having the processing circuit and IC chip terminals 506 provided thereon. This structure prevents sensor 501 from having a small overall height.
In addition, sensor chip 502 is physically joined directly to both a lower surface of IC chip 503 and the upper surface of protective case 504 with adhesives 507a and 507b, respectively. Therefore, stresses resulting from curing of adhesives 507a and 507b remain built up around support body 502c and flexible portion 502a of sensor chip 502. A thermal stress may be developed due to a difference in linear expansion coefficients of protective case 504 and sensor chip 502. Furthermore, another stress is exerted directly on support body 502c of sensor chip 502 if a printed circuit board having protective case 504 mounted thereon deforms. These stresses can degrade a temperature characteristic of the sensor output, cause hysteresis in the sensor output and the like, thereby decreasing accuracy of sensor 501 for detecting accelerations.